X-ray detectors comprising a directly converting semiconductor layer enable individual X-ray quanta to be detected quantitatively and energy-selectively. In the case of this type of X-ray detectors, an incident X-ray quantum generates in the semiconductor layer, on account of in part multistage physical interaction processes with a semiconductor material, free charge carriers in the form of electron-hole pairs. By way of example, semiconductor materials in the form of CdTe, CdZnTe, CdTeSe, CdZnTeSe, CdMnTe, InP, TIBr2 or HGI2are suitable for detecting X-ray quanta since these materials have a high X-ray absorption in the energy range of medical imaging.
In order to detect the quantum absorption events corresponding to an X-ray quantum, electrodes are fitted to the two sides of the semiconductor layer, a voltage being applied to the electrodes in order to generate an electric field. For the spatially resolved detection of the absorption events, one electrode is embodied in pixelated fashion and is designated as a read-out electrode. The electrode arranged opposite it is usually embodied in planar fashion and is designated as a counterelectrode. In the electric field generated, the liberated charge carriers are accelerated depending on type of charge and polarity to the electrodes, where they induce electrical signals in the form of currents. The currents are converted, by means of an evaluation unit, into an evaluation signal, the magnitude of which is proportional to the area integral of the current curve and thus proportional to that quantity of charge which is liberated by an incident X-ray quantum. The evaluation signal thus generated is subsequently conducted to a pulse discriminator, which, in a threshold-value-based manner, detects the X-ray quanta in a counting manner and/or energy-selectively.
A prerequisite for an error-free detection of X-ray quanta is a calibration of the X-ray detector that involves defining suitable threshold values. In this case, the threshold values are chosen such that a signal generated by noise does not initiate detection of a purported X-ray quantum, and that, in the case of an energy-selective detection, an energy or an energy range can be assigned to the individual X-ray quanta. Inhomogeneities in the material of the semiconductor layer make it necessary to carry out the calibration in spatially resolved fashion. In a first known calibration method, use is made of a radioactive preparation for irradiating the X-ray detector. In this case, the radioactive preparation liberates X-ray radiation having a known defined energy. Noise and energy thresholds for the pulse discriminator are determined by means of an evaluation of the detected electrical signals and of the evaluation signals derived therefrom. On account of in part legally established stipulations for the handling of radioactive preparations, this method is suitable only for a single calibration in the laboratory prior to assembly. However, it is not practicable in the assembled X-ray apparatus.
Since the material behavior of the semiconductor layer can vary over time and under the action of X-ray radiation and it is also necessary to take account of a drift of the evaluation electronics, it is necessary to repeat the calibration at certain time intervals and in an optimum case directly before the beginning of measurement. A calibration using X-ray radiation before the beginning of measurement is not considered since the patient would in this case be exposed to an additional X-ray dose. In a second known calibration method, for the purpose of repeatedly performing the calibration without the action of X-ray radiation, electrical signals are coupled capacitively onto the read-out electrode. This is then using a pulse generator that is electrically contact-connected to the counterelectrode. The pulse generator generates variable charges on the counterelectrode in such a way that electrical signals coupled in capacitively on the read-out electrode are generated with a pulse shape such as would be expected upon the incidence of an X-ray quantum having a specific energy.
Taking this as a departure point the intention is to provide an X-ray detector comprising a directly converting semiconductor layer and a calibration method for such an X-ray detector which provide in an improved form the prerequisites for a calibration of the X-ray detector that is repeatable directly before the beginning of measurement.